Neuroblastoma is the most common malignant disease of early childhood. Infants younger than 1 year have a good prognosis, while older patients show metastatic neuroblastoma with poor outcome and death. Because current treatment strategies are ineffective in children older than 1 year, innovative therapeutic strategies are urgently needed. This proposal aims to develop a combination therapy to control the growth of malignant neuroblastoma. Our preliminary results indicate that retinoids, such as all-trans-retinoic acid (ATRA), 13-cis-retinoicacid (13-CRA), and fenretinide [N-(4-hydroxyphenyl) retinamide or 4-HPR] induce differentiation in 5 human neuroblastoma cell lines (SH-SY5Y, SK-N-BE2, SK-N-DZ, IMR-32, and SK-N-MC). Also, flavonoids, such as apigenin (APG), (-)-epigallocatechin (EGG), (-)-epigallocatechin-3-gallate (EGCG), and genistein (GST), induce apoptosis. Interestingly, neuroblastoma cells differentiated with specifically 4- HPR became more sensitive to flavonoids, especially GST, for induction of apoptosis with increased ratio of bax:bcl-2 and upregulation of calpain and caspases. Treatment of neuroblastoma xenografts in nude mice with a combination of low doses of 4-HPR and GST regressed tumors with induction of differentiation and apoptosis. We hypothesize that (1) 4-HPR induces differentiation and down regulates survival factors;(2) GST induces apoptosis inhibiting survival factors and activating cysteine proteases and enhances apoptosis in differentiated cells;(3) treatment of neuroblastoma xenografts with 4-HPR and GST induces differentiation and enhances apoptosis. Our central hypothesis is that promotion of differentiation with a selective retinoid down regulates telomerase and survival proteins, and sensitizes differentiated cells to a selective flavonoid for enhancing apoptosis with activation of cysteine proteases. The objective is to use a combination therapy for differentiation and apoptosis in neuroblastoma cells in culture as well in ectopic and orthotopic xenografted animal models. To achieve this objective, we propose the following specific aims: Specific Aim 1: To determine the morphological and biochemical features of neuronal differentiation in five human neuroblastoma cell lines in response to selected retinoids. Specific Aim 2: To examine the induction of apoptosis in five human neuroblastoma cell lines following treatment with selected flavonoids and enhancement of apoptosis in differentiated cells. Specific Aim 3: To determine the efficacy of combination of a selected retinoid and a selected flavonoid for induction of differentiation and enhancement of apoptosis in neuroblastoma in ectopic and orthotopic xenografts in athymic nude mice. Success of our therapeutic strategy can form a basis for treatment of neuroblastoma in humans.